222 THE PHENOMENON OP 



of all refer again to Hydrodidyon as an example. The 

 reason why the mode of formation of the new nets in the 

 cells of the old one has hitherto been so rarely and so 

 inaccurately observed,* doubtless arises from the time at 

 which this process commences. If a single fully-developed 

 net of this plant, or even mere fragments of nets, are 

 placed in a shallow saucer of water, we may almost cer- 

 tainly reckon upon finding fully-formed young nets in some 

 of the old cells on the next, or at all events on the second 

 morning, and these in cells which exhibited no alteration 

 whatever.! If we wish to see the origin of these young 

 nets, we must not lose the earliest hours of the morning, 

 for the tremulous movement commencing after the forma- 

 tion is complete, and the final union of the gonidia into 

 a net (see p. 137) takes place shortly after sun-rise, in 

 the middle of summer between four and five in the 

 morning, at the end of summer between six and eight 

 o'clock, and only in dull days of autumn, in which, 

 however, the formation of new nets rarely occurs, some- 

 times as late as ten o'clock in the morning. The 

 swarming out of the microgonidia of Hydrodidyon 

 always takes place rather later in the day than the 

 formation of the nets, in summer usually between seven 

 and nine, in autumn between ten and two o'clock. As 

 the swarming lasts several hours, the active condition may 

 often be observed late in the afternoon (sometimes at five 

 o'clock in the evening), j 



* The most recent treatise on this subject, by Morren ('Mem. de I'Acad. 

 Key. de Bruxelles,') xiv, (1841,) is an almost incom])rehensible tissue of errors. 



f In deeper vessels of water the propagation does not commence so soon, 

 which is certainly explained by the diminished contact with the atmosphere, 

 especially with the oxygen required for the process of solution. 



% In regard to the relation of the occurrence of the swarming-out 

 microgonidia to the net-forming macrogonidia, I may add that it appears to 

 depend, in part, upon external circumstances. Many days I saw only net- 

 formation, in others both net-formation and swarming, on others, again, espe- 

 cially on dull and rough days, formation of swarmers in unususd quantity 

 unaccompanied by any net-formation. It is strange that the formation of 

 swarmers, on the whole more frequent than the formation of new nets, has 

 not been more frequently observed by earlier investigators. Treviranus saw 

 them but did not observe their origin. ('Beitrage zur Physiol.,' p. 81.) 



